The healthy would have a better chance of surviving an epidemic if they could identify its first victims -- while they are still looking well -- and steer clear. Unfortunately, those early churnings of the immune system are far too subtle to detect. Or maybe not.

We have long known that bacteria living on and in our bodies outnumber our cells -- which themselves sum up to a number that exceeds any hope of real understanding -- by 20 to one. We are a rounding error in our own skin.